guarantee
noun
a written assurance that some product or service will be provided or will meet certain specifications
an unconditional commitment that something will happen or that something is true
“there is no guarantee that they are not lying”
a collateral agreement to answer for the debt of another in case that person defaults
Synonymsguaranty
verb
give surety or assume responsibility
“I vouch for the quality of my products”
Synonymsvouchmake certain of
“This nest egg will ensure a nice retirement for us”
“Preparation will guarantee success!”
promise to do or accomplish
“guarantee to free the prisoners”
Synonymsundertakestand behind and guarantee the quality, accuracy, or condition of
“The dealer warrants all the cars he sells”
“I warrant this information”
Synonymswarrant
Source: WordNet 3.1, Princeton University